Tissue paper is a staple of everyday life. Tissue paper is frequently used for bath tissue, facial tissue, and for paper towels. Tissue paper is typically cellulosic and fibrous, the fibers having been prepared from various types of pulping processes and various species of deciduous and coniferous woods. Recycled and synthetic fibers may also be used.
Tissue paper needs to have properties which meet three primary criteria in order to be consumer acceptable: bulk which encompasses the inversely related properties of caliper and density, strength and softness. Typically softness and strength have been optimized by layering the fibers comprising the paper. Bulk is obtained in other manners, such as molding a wet embryonic web into a photopolymer belt, according to commonly assigned U.S. Pat. No. 4,637,859, issued Jan. 20, 1987 to Trokhan, the disclosure of which patent is incorporated herein by reference.
The art has used both homogeneous blends of such cellulose fibers and stratified layers of such fibers in papermaking. Particularly, separation of the various types of fibers into various layers during papermaking has been attempted several times in the prior art.
As used herein, the term "layer" refers to a stratum of fibers formed in the wet end of a papermaking machine and which is different from an adjacent stratum. The difference may include the preponderance of fiber species. The difference may also include the chemical treatment, pulping or any other factors which affect the properties of that stratum of fibers.
The layer is typically deposited onto a forming wire. Particularly, a layer refers to an individual stratum of fibers, typically superimposed on one or more different strata, and deposited onto a common forming wire with the other strata. Deposition is accomplished by having each layer deposited from a separate channel of a split headbox. Alternatively, the layers can be deposited onto a common forming wire from separate headboxes.
Another way to determine if paper is layered is to analyze various regions of the paper throughout its thickness. Preferably the analysis includes the regions disposed on the opposed faces of the paper and several regions between the opposed faces of the paper.
Different regions of the paper throughout its thickness may be accessed for analysis by removing fibers from the paper, starting at either or both faces with adhesive tape, as is known in the art. The fibers adhering to the tape are then analyzed for species content, chemical treatment, pulping, or any other factor which may affect the properties of the fiber.
If 1) at least 25 percent of the fibers at a given region of the paper are different than the preponderance of fibers at another region of the paper having a different position within the thickness of the paper, the paper is considered to be layered, or 2) the paper is made as described above using a split headbox or more than one headbox which discharges onto a common forming wire, the paper is considered to be layered.
In contrast, a "ply" refers to a single sheet, having one or more layers, as taken off of the forming element. A ply may later be combined with other plies, embossed, etc., as desired.
U.S. Pat. No. 2,881,669, issued Apr. 14, 1959 to Thomas et al. discloses paper having three layers. The three layers comprise a central layer of short fibers and two outer layers of long fibers. This arrangement is said to improve strength and provide reduced ply separation. Papers subject to ply separation, i.e. having layers which shear during creping, are said to be disclosed in U.S. Pat. No. 4,166,001, issued Aug. 28, 1979 to Dunning et al. Dunning et al. discloses a three layered paper having a central layer of weakly bonded fibers between two outer layers of strongly bonded fibers. Ply separable paper is also disclosed in commonly assigned U.S. Pat. No. 4,225,382 issued Sep. 30, 1980 to Kearney et al. Kearney et al. teaches a three layered structure having two layers of well bonded fibers separated by an interior layer of short fibers.
Commonly assigned U.S. Pat. No. 3,994,771, issued Nov. 30, 1976 to Morgan, Jr. et al. extends the concept of layered paper to through air drying.
European Patent Application 0 531 112 A1, published Mar. 10, 1993, in the name of Jujo discloses a three layer toilet paper. The intermediate layer is about one hundred percent wood pulp. The outer layers comprise wood and rayon pulps mixed in a weight ratio of 40-80:60-20. This arrangement is said to provide a strong, absorbent toilet paper which is also water dispersible.
U.S. Pat. No. 5,087,324, issued Feb. 11, 1992 to Awofeso et al. discloses a two-ply, two-layer paper towel having two outer layers and two inner layers. The two outer layers comprise blends of Kraft softwood and Kraft hardwood. The two inner layers comprise high bulking fibers or chemi-thermomechanical pulp.
Typically the prior art has used two-layer paper, joined together in a two-ply laminate. However, a two-layer paper, even having two plies, still does not have the degrees of freedom necessary to optimize bulk, softness and strength. For through air dried paper, two ply production economics are less attractive than single ply production economics.
The prior art also teaches a single ply, three layer paper. The two outer layers provide softness. The inner layer provides strength.
None of these prior art attempts have overcome the problem of increasing caliper, while maintaining adequate strength. Typically, as the caliper of the paper increases, density decreases. This decrease in density, unfortunately, causes a loss in tensile strength which is not overcome by the prior art attempts at layering. In addition to overcoming the tradeoff between caliper and tensile strength, there still must be a consumer-perceived tactile softness in the layered paper.
Accordingly, it is an object of this invention to provide a layered tissue which has both relatively high caliper and relatively high tensile strength. It is further an object of this invention to provide a paper which provides a soft tactile sensation to the user.